Background: In Kenya, HIV prevalence remains high among sex workers (SW). Kenya''s Fourth National AIDS Strategic Plan includes scaling up targeted oral pre-exposure prophylaxis (PrEP) for key populations. To support the Kenyan government in its plans to roll out this new intervention, USAID- and PEPFAR-funded Health Policy Project (HPP) and the Bill & Melinda Gates Foundation-funded Kenya PrEP Demonstration Project conducted prospective costing of oral PrEP services for SW in Kenya to be provided by the Sex Worker Outreach Program (SWOP), a network of HIV/STI prevention and care clinics for sex workers.Methods: We developed ''bottom-up'' and ''top-down'' costing methodologies to assess provider costs for a comprehensive package of PrEP services for SW through SWOP clinics. ''Bottom up'' costs were estimated from facility-based data and through process flow diagrams based on PrEP service delivery. ''Top-down'' costs allocated 2012 costs for each cost category based on the anticipated proportion of PrEP client visits (except ARVs and lab costs, which were costed on an ingredients basis, and projected costs for additional PrEP-specific items such as equipment and training). From January-February and June-August 2013, the study teams collected data from SWOP clinic financial records and asset registers, Kenya Medical Supplies Agency, and interviews.Results: The ''bottom-up'' cost per client-year to provide a comprehensive package of PrEP services was US$602. The ''top-down'' cost per client-year was US$597 in Year 1 and US$408 in Year 2. Major cost drivers for both methods include labor, antiretroviral drug costs, and laboratory tests.Conclusions: Our findings provide the first evidence-based cost estimates for PrEP provision for SW in Kenya. The two approaches led to similar costs per client year of PrEP. In comparison, another model created by Andrew Hastings, which is based on assumptions rather than actual cost data, found similar costs per client in its higher range estimates. These results will be incorporated into cost-effectiveness analyses and used by the Government of Kenya to help make strategic decisions about combination HIV prevention programs. Additional cost studies of government and other NGO service providers will be needed to understand the impact of introducing PrEP on the overall HIV/AIDS budget in Kenya.